1. Field of the Invention
This invention relates to Brayton and Ericsson open cycle heat engines where the engine cycle comprises the steps of Ericsson (isothermal) compression, recuperative heat addition, Brayton (adiabatic) expansion, and recuperative heat removal. More particularly, it relates to a commercially viable, open cycle, positive displacement engine where heat addition to the cycle is effected solely through a recuperator by burning fuel in the expanded, low pressure, exhaust stream.
2. Description of Prior Art
The increasing world-wide demand for electrical and mechanical power production, combined with concern for the environment, has led to the need for new, practical, engines that can cleanly and efficiently produce that power from combustion of a wide variety of fuels.
Internal combustion engines are well developed but require highly refined liquid or gaseous fuels—fuels that are generally limited in supply and that have their primary sources in politically unstable regions. Furthermore, the combustion process in internal combustion engines, from ignition to extinction, must take place in hundredths of a second. Additional constraints to clean and efficient internal combustion result from the relatively fixed geometry of the combustion chamber and the need to provide smooth, detonation free, flame propagation. These constraints severely compromise the combustion process and lead to incomplete combustion that generates undesirable exhaust emissions.
External combustion engines offer impressive advantages over internal combustion engines. External combustion engines can accommodate a wide variety of fuels, in any phase, and without regard for detonation (knock) characteristics. They can use low pressure, continuous, combustion processes that allow long combustion times for maximum efficiency and minimum exhaust emissions. In turn, low pressure combustion easily incorporates catalytic burners, re-circulating bluff body flame holders, rich/lean staged combustion burners, and related leading edge technologies that are now being developed to provide nearly complete combustion with minimal harmful exhaust pollutants.
Finally, and importantly, low pressure external combustion uniquely lends itself to recovery of otherwise wasted exhaust heat for significant efficiency improvement through use of a counterflow heat exchanger to preheat the combustor air supply with the hot exhaust gas.
Just as engines can be defined as being internal combustion or external combustion, they can also be classed as closed cycle engines and open cycle engines. Closed cycle engines, such as steam engines and Stirling engines, use the same working fluid over and over to generate power by adding and removing heat through heat exchangers. Open cycle engines simply use air as the working fluid. The engine takes air in and exhausts air out as part of the power generation process. Open cycle engines have advantages over closed cycle engines in simplicity, cost, and efficiency because the air used in the power cycle can also be used in the combustion process to yield an integrated engine/combustion process that is both simple and efficient. The advantages of open cycle engines over closed cycle engines have caused steam engines to become increasingly obsolete and have prevented Stirling engines from becoming commercially viable.
From the previous paragraphs it would seem that an external low-pressure combustion, open cycle engine (ELPC/OC engine) would combine the best features to produce an optimal engine. However, at this time, there are no commercially successful ELPC/OC engines on the market. The reason is, although such an engine seems straightforward, the prior art has all encountered practical limitations.
The most promising prior art ELPC/OC engine is described in U.S. Pat No. 5,894,729 (“Afterburning Ericsson Cycle Engine”, Proeschel, 1997). The Afterburning Ericsson Cycle (AEC) engine has all the ELPC/OC advantages of: being able to utilize a wide variety of fuels; having continuous, low pressure, combustion; and integrating the engine and combustor so that the combustion air is preheated by the exhaust. In addition, being based on the Ericsson cycle, the AEC has the potential for very high thermodynamic efficiency.
The AEC engine comprises a compressor having cooling provisions to allow it to approximate isothermal compression, a counterflow heat exchanger (recuperator) for heating the compressed air with heat recovered from the engine exhaust, an expander with heating passages to approximate isothermal expansion, and one or more afterburners in the expander exhaust that provide heat to the expander heating passages and to the recuperator.
The temperature entropy diagram of FIG. 1 shows the ideal AEC engine cycle. The cycle consists of:                Point 1 to Point 2: Isothermal compression at ambient air temperature, Tc, from low pressure Po to high pressure P1.        Point 2 to Point 3: Constant pressure recuperated heating from Tc to Th.        Point 3 to Point 4: Isothermal expansion, at Th,        Point 4 to 5 and Point 4a to 5a: Constant pressure combustion heating.        Point 5 to 4a and Point 5a to 4b: Constant pressure cooling in heat transfer passages to provide the heat needed for Point 3 to Point 4.        Point 4b to Point 1: Constant pressure recuperated cooling from Th to Tc.        
The cycle of FIG. 1 has the efficiency of a Carnot cycle operating between Tc and Th. Since the Carnot cycle defines the maximum possible thermodynamic efficiency, the AEC is a very promising cycle.
At first it would seem that making a practical AEC engine would depend on a high level of success in achieving nearly isothermal expansion from Point 3 to Point 4. Surprisingly, in developing the AEC engine, it was found that, particularly at pressure ratios (P1/Po) less than about 6, the cycle efficiency was almost independent of the effectiveness in approaching ideal isothermal expansion.
FIG. 2 shows the predicted brake shaft efficiency of a typical prototype AEC design as a function of pressure ratio and expander heating effectiveness. (Expander heating effectiveness is the ratio of the actual heat transfer rate to the rate required for isothermal expansion.) The FIG. 2 results are for a constant recuperator inlet temperature (Point 4b in FIG. 1) of 816° C. (1500° F.) and include the effects of heat losses, pressure losses (particularly in the expander heat transfer passages), and mechanical losses.
The AEC engine efficiency is not strongly affected by expander heating effectiveness for two reasons. First, obtaining high expander heating effectiveness requires long and highly finned expander heating passages. The fins cause flow restriction and a high backpressure. Overcoming the high backpressure costs much of what is gained by heating the expander. Second, the heat that cannot be transferred to the expansion process is still available to the cycle through the recuperator process (Point 4b to Point 1). With a high recuperator effectiveness (93% in this case) high engine cycle efficiency is still obtainable.
FIG. 3 shows the required peak combustion temperatures corresponding to the same conditions as FIG. 2. Higher combustion temperatures are needed to provide the higher heat transfer rates for higher values of expander heating effectiveness. However, the higher combustion temperatures are undesirable because they increase the amount of nitrogen oxides (NOx) produced from the combustion process and because they increase engine thermal stress.
The AEC development results of FIG. 2 and FIG. 3 show there is a strong case for simplifying the AEC engine by doing away with the expander heating passages (corresponding to the case of zero expander heating effectiveness). Construction is simplified, peak temperatures are reduced, and, with practical pressure ratios, the engine efficiency is essentially unchanged.
Eliminating the expansion heating from FIG. 2 results in the ideal cycle of FIG. 4. The expansion process, Point 3 to Point 4, is adiabatic or isentropic. A single heating process then heats the air to the recuperator inlet temperature, Th, at Point 5. The exhaust heat from Point 5 to Point 1 is transferred through the recuperator to provide the heat for Point 2 to Point 3.
U.S. Pat. No. 2,438,635 (“Turbine System Utilizing Hot Driving Gases”, Haverstick, 1948) teaches a turbine system roughly operating according to FIG. 4. However, Haverstick's patent includes the additional and counterproductive complexity of splitting the exhaust flow in two and introducing the second half of the flow at an intermediate point in the recuperator.
U.S. Pat. No. 3,621,654 (“Regenerative Gas Turbine Power Plant”, Hull, 1971) covers almost all possible combinations of recuperated Brayton cycle engines, including engines operating on the cycle of FIG. 4. However, Hull teaches turbine machines for the compression and expansion processes. Turbine engines are viable for large powerplants but do not work well for smaller powerplants. Blade edge losses are difficult to control with smaller size turbines and the high turbine speed makes integration with electrical generators difficult. Also turbine engines cannot be built or maintained in small local machine shops whereas positive displacement engines, particularly in micro-generation sizes, can easily be built and maintained in automotive machine shops.
U.S. Pat. No. 3,893,300 (“External Combustion Engine and Engine Cycle”, Connell, 1975) teaches an engine operating on the FIG. 4 cycle with a positive displacement compressor and a turbine expander. Connell recognizes the limitations of small turbines for the compression process but still teaches a turbine for the expansion process. Furthermore, Connell teaches the need for heat storage means to facilitate rapid response to load changes. He failed to appreciate that an actual recuperator capable of achieving the high heat transfer effectiveness needed to achieve high engine efficiency will inherently have substantial thermal storage capability. Connell's heat storage means is therefore unnecessary and can be omitted without loss of capability.
U.S. Pat. No. 3,756,022 (“External Combustion Engine”, Pronovost et. al., 1973) teaches an engine operating roughly according to FIG. 4 having a positive displacement, reciprocating, expander. However, Pronovost's invention is inoperative because he failed to appreciate the key needs for cooling the compressor, insulating the exparider, and protecting the reciprocating expander seals and mechanisms from high temperature. He also teaches a combined combustor/recuperator or “heating chamber” which acts as a cross flow heat exchanger. Pronovost did not understand that a high effectiveness counterflow recuperator is another key requirement to make this type of engine a practical commercial success.
It is the primary aim of this invention to overcome the disadvantages of current ELPC/OC engines discussed above and to achieve a practical, commercially successful ELPC/OC engine having high efficiency, low emissions, ease of control, and economy of manufacture by implementing the several objects listed below.